1. Field of the Invention
The present invention relates in general to a flame detecting apparatus used in a combustion control system for a water heater or the like which is provided with a gas burner or the like. In particular, the invention concerns a flame detecting apparatus which includes a pressure switch serving as means for detecting presence or absence of flame and a voltage comparison circuit serving as means for determining the presence or absence of flame on the basis of the input signal supplied by the pressure switch.
2. Description of the Prior Art
In the burner in which the pressure switch is employed as the means for detecting the presence or absence of flame, a considerably high pressure prevails within a combustion chamber. As a typical example of such burner, there can be mentioned a pulse burner.
The pulse burner differs from the burner of general type where the flame is continuously maintained independent of time lapse in respect that the flames are produced intermittently in a pulse-like manner. Various types of the pulse burners have been heretofore known. However, in general, the pulse burner includes a combustion chamber, a valve for controlling the supply of gaseous fuel and air to the combustion chamber and an exhaust conduit which is designed as to cooperate with the combustion chamber to produce vibration of gas column at a predetermined frequency. The combustion chamber and the exhaust conduit serve as a heat exchanger. In operation, the fuel-air mixture is supplied to the combustion chamber through the control valve and ignited by an ignitor to burn explosively. Under the pressure thus produced by the explosive combustion, the control valve is forcibly closed, while the gaseous combustion products are discharged through the exhaust conduit. This results in that a vacuum (negative) pressure prevails within the combustion chamber, whereby the valve is again opened to feed the fuel-air mixture to the combustion chamber. In this way, the vibration of gas column is produced within the combustion chamber and the exhaust conduit. Under the action of the vibration of gas column, the after-burning (or remnant) flame or high temperature gas is compelled to be fed back to the combustion chamber to serve as an ignition source for triggering the explosive combustion of the new charged fuel-air mixture. The resultant increased pressure again forces to discharge the combustion product gas through the exhaust conduit. This cycle is successively repeated. In this manner, the combustion or burning takes place intermittently, that is, in the pulse-like manner.
The circuit provided internally of a combustion control apparatus used in combination with the pulse burner described above has to be imparted with a fail-safe feature so that short-circuit fault produced at any given time can not lead to dangerous state or situation.
The combustion control apparatus includes generally a voltage comparison circuit as the means for determining the presence or absence of the flame. This type of combustion control apparatus may be constituted, for example, by an integrated circuit element commercially available from Hitachi, Ltd. in Japan under the trade name "HA-16605W".
The pressure switch which is closed in response to the detection of flame is so combined with the voltage comparison circuit that when the pressure switch coupled to the inverting input terminal of the voltage comparison circuit is closed (i.e. when the flame is present), the voltage comparison circuit outputs a high level signal. Accordingly, if a short-circuit fault occurs, for example, across a resistor connected in parallel with the pressure switch in the open state of the pressure switch, the state equivalent to the closing of the pressure switch is brought about to cause the voltage comparison circuit to output the high level signal, involving erroneous operation.
Further, since a relatively low voltage is applied across the pressure switch with the result that electric fields produced in contacting surfaces of the switch contacts are feeble, erroneous operation of the combustion control apparatus may also take place due to the contact failure.
In a flame-rod type flame detecting apparatus, a voltage comparison circuit having a negative offset voltage is made use of, wherein the non-inverting input terminal of the voltage comparison circuit is connected only to the ground (earth) with an attempt to prevent the erroneous operation of the voltage comparison circuit due to the short-circuit fault of the resistor.